1. Field of the Invention
The present invention relates to a cryogenic system for cooling a superconducting magnet or the like.
2. Description of the Related Art
In conventional apparatuses including superconducting magnets, in order to retain their superconducting properties, it has been necessary to maintain the magnets at a cryogenic temperature close to absolute zero. Examples of a technique for maintaining a magnet at a cryogenic temperature include a technique in which a superconducting magnet is immersed in a coolant such as liquid helium and a technique for directly cooling a superconducting magnet using a cryogenic refrigerator.
FIG. 8 shows a configuration of a known cryogenic system 101. This figure shows a cross section of an MRI system (a magnetic resonance imaging scanner) known as a medical instrument. This system includes a solenoidal superconducting magnet 103 of which the central axis extends horizontally.
The cryogenic system 101, which is of such a type that helium gas is recondensed, includes a containment vessel 102 in which the superconducting magnet 103 is housed in such a manner that the superconducting magnet 103 is immersed in a coolant 104 (liquid helium), a vacuum vessel 105 having a vacuum space surrounding the containment vessel 102, and a thermal shield 106 which is placed in the space so as to surround the containment vessel 102 and which has a function of reducing the amount of radiation heat transferred from the vacuum vessel 105 to the containment vessel 102.
The cryogenic system 101 further includes a tubular refrigerator sleeve 107 which extends through the vacuum vessel 105 and the thermal shield 106, which has a base section communicatively connected into the containment vessel 102, and which has an opening facing outward. The cryogenic system 101 also includes a refrigerator 108, placed in the refrigerator sleeve 107, for recondensing coolant gas generated from the coolant 104 at cryogenic temperature.
Since the refrigerator sleeve 107 and the containment vessel 102 are communicatively connected to each other, an upper zone of the containment vessel 102 and the refrigerator sleeve 107 are filled with the coolant gas of which the pressure is equal to the saturation vapor pressure at a temperature at which the superconducting magnet 103 is operated.
The refrigerator 108 is tubular and long and has two stages: a first cooling stage 119 placed at a middle portion and a second cooling stage 121 placed at an end portion. The first cooling stage 119 is thermally connected to the thermal shield 106 and the second cooling stage 121 is thermally connected to a recondenser 130 (fins). The refrigerator 108 has a cooling capacity sufficient to maintain the second cooling stage 121 at cryogenic temperature (about 4 K). Therefore, the surface temperature of the recondenser 130 is maintained lower than the temperature of the coolant; hence, the vapor of the coolant 104 can be condensed into liquid by making the vapor in contact with the recondenser 130. According to this configuration, it is not necessary to refill the cryogenic system 101 with the coolant 104 as long as the refrigerator 108 can function.
For the refrigerator 108, in order to perform periodic maintenance, the operation is stopped in some cases. During maintenance, the refrigerator 108 is drawn out of the refrigerator sleeve 107 and a new refrigerator 108 is inserted into the refrigerator sleeve 107 and then started up. The operation is continued until the new refrigerator 108 reaches a steady state.
During the maintenance operation, a serious problem described below can arise.
When the refrigerator 108 is drawn out of the sleeve, air and/or moisture of which the amount is equal to the volume of the refrigerator 108 enters the sleeve from outside in some cases. The resulting air and/or moisture is instantaneously condensed in the refrigerator sleeve 107 and fixed thereto. This is because the inside of the refrigerator sleeve 107 is maintained at a temperature lower than outside air temperature, that is, the temperature of the thermal shield 106 is usually 30 to 60 K and the temperature of the bottom of the refrigerator sleeve 7 is 3 to 5 K.
The following problem arises: a problem in that the heat conduction between the thermal shield 106 and a first cooling stage 119 of the new refrigerator 108 is deteriorated due to the fixed air (oxygen and nitrogen) and/or moisture and the refrigerator 108 cannot therefore be operated at full power.
Therefore, when the conduction between the first cooling stage 119 and the thermal shield 106 is low, the contact thermal resistance becomes large. This causes an increase in the temperature of the thermal shield 106 to increase the amount of heat transferred to the containment vessel 102. At worst, the amount of heat transferred to the containment vessel 102 exceeds the liquefaction capacity of the recondenser 130; hence, the whole of the vapor of the coolant 104 cannot be liquefied even if the refrigerator 108 is operated.
Various attempts have been made to achieve an object that the intrusion of air is prevented and the thermal resistance between the refrigerator 108 and the coolant 104 is reduced. In a technique disclosed in, for example, Japanese Unexamined Patent Application Publication No. 5-223379, an expandable, flexible maintenance bag is attached to the proximal end (an external portion of the system) of the refrigerator sleeve 107, deflated by exhausting air from the bag using an exhaust blower, and then inflated by introducing a coolant gas into the bag through a gas line such that an atmosphere filled with the coolant gas is formed. The refrigerator 108 is subjected to maintenance in the atmosphere.
However, it is troublesome to use the technique disclosed in Japanese Unexamined Patent Application Publication No. 5-223379 because the maintenance bag must be attached before the refrigerator 108 is replaced and because the exhaust blower for exhausting air from the bag and other tools must be prepared.
In addition, the replacement is performed in such a manner that an operator inserts his or her hands in a pair of gloves extending in the maintenance bag; hence, this operation is troublesome. The replacement is interrupted in some cases because the maintenance bag is broken.